1. Field of the Invention
The present invention relates to an AGC circuit preferably applied to a receiver of digital voice broadcast.
2. Description of the Related Art
Digitization of broadcasts have progressed and also a transmission system of the broadcast wave is being shifted from a conventional analog system such as AM or FM to a digital system. Particularly, in Europe, digital broadcast of voice programs has been started and as the digital voice broadcast, there is DAB (digital voice broadcast complying with Eureka147 standard).
According to DAB, by adopting OFDM as a modulation system having a transmission band of about 1.5 MHz and layer II of MPEG audio for data compression of an audio signal, a maximum of 64 sets of digital audio signals or digital data are simultaneously broadcast.
In this case, OFDM is a multiplex carrier system for carrying out broadcast by simultaneously transmitting a number of carriers. For example, according to a broadcast mode referred to as DAB mode II, 384 carrier signals are transmitted at intervals of 4 kHz within the above-described transmission band. Further, frequencies used in broadcast fall in a range of 170 MHz through 220 MHz and 1.4 GHz band.
In the meantime, according to a DAB receiver mounted on a moving body such as a car, an intensity of the radio wave is changed over time by fading and a state of the fading is changed by a moving speed of the moving body, a received frequency and so on. Therefore, in order to provide low bit error rate in the DAB receiver, an AGC circuit is needed and a time constant thereof needs to change into a pertinent value in correspondence with a situation of receiving the broadcast radio wave such as the radio wave intensity.
Further, capacitors for the time constant are externally attached to ICs for AGC and depending on the DAB receiver, the capacitors for the time constant are switched by a switching circuit in accordance with a situation of receiving the broadcast radio wave.
However, in this case, the time constant can only be changed in steps, further, the number of parts is increased.